The present invention relates to a developing apparatus adapted to a two-component developing method for use in an electrophotographic method employed in a copying machine, a printer, a facsimile machine or the like. More particularly, the present invention relate to an improvement in control of the density of toner by using a toner-density sensor for magnetically detecting the density of toner and a technique for detecting non-loading of a developer container and a failure of the toner-density sensor by improving the control of the density of the toner.
A conventional method of detecting the density of toner in an image forming apparatus which employs the two-component developing method using toner and a carrier has generally employed a method of magnetically detecting change in the magnetic permeability of the developer. In a case of monochrome images, magnetic toner and a magnetic carrier are employed, while non-magnetic toner and the magnetic carrier are employed in a case of color images. A fact that the mixture ratio of the toner and the carrier is changed and thus the magnetic permeability is changed is used such that the magnetic permeability of the developer is measured. Thus, the density of the toner is detected.
In the foregoing case, an output of, for example, a magnetic-permeability sensor denoting the detected density of the toner is compared with a predetermined reference value. To make the output denoting the detected density of the toner to be the same as the reference value, toner is supplied. Thus, control is performed in such a manner that the density of toner is constant.
However, the developer composed of toner and carrier cannot uniformly be stirred with facility. The developer remains in the vicinity of the detecting surface of the toner-density sensor or toner adheres to the detecting surface of the toner-density sensor after an elapse of long time. Thus, an error occurs in detecting the density of toner.
If an error in detecting the density of toner occurs, the control of the density of toner cannot correctly be performed. Thus, a required density of toner in the developer cannot be realized. Hence it follows that a problem arises in that the print density is lowered, toner is splattered and an undesirable state of the carrier is realized.
To overcome the foregoing problem, a method has been suggested in Japanese Patent Publication JP-2-64560. According to the suggestion, the toner which remains adjacent to the detecting surface is reliably raked out by a mechanical means to exert periodic vibrations on the developer. Moreover, the component of the vibration waveform is superimposed on the output of the toner-density sensor obtainable when no vibration is exerted. Thus, a comparison is made with a reference control level to form a pulse. Thus, the density of toner can be controlled for a long time without remaining of the developer adjacent to the detecting surface.
With the foregoing method, control is performed by using the formed pulse such that the output value, which has intentionally be superimposed, is binary-coded. Therefore, the dynamic range is narrowed excessively. If the dynamic range is widened, the inclination of the period of the vibrations must be moderated. Hence it follows that adjustment must be performed because of joining of the toner-density sensor and dispersion.
Hitherto, a color image forming apparatus has been known which forms a color image by using a plurality of different developer containers in which four different toner materials are enclosed. The four types of the developer containers have been structured such that each developer container is removable when inspection is performed in order to easily perform maintenance and the inspection.
Since the plural developer containers are provided, failure of resetting easily occurs when the container is loaded to perform the maintenance and inspection. To overcome the foregoing problem, a technique has been disclosed in Japanese Patent Publication JP-6-51625. That is, an apparatus is used which reshapes a detection signal supplied from the toner-density sensor into a pulse signal. In accordance with the number of generated pulse signals during a predetermined gate period, the apparatus detects the density. Thus, whether or not the developer container has been loaded is detected by making a determination that the developer container has not been loaded if no pulse signal is generated in the predetermined gate period.
However, the foregoing technique is arranged to simply detect whether or not the pulse signal has been generated in the gate period. If a stirring member of the developer container is broken down owing to stress of the developer or the like, the structure, in which the pulse signal is electrically reshaped, undesirably generates the pulse signal because the developer has been detected. Therefore, the stirring cannot sufficiently be performed. As an alternative to this, a determination is undesirably made that the developer container has not been loaded if the toner sensor is broken down. Hence it follows that accurate detection cannot be performed.
In recent years an image forming apparatus using an a-Si photosensitive material and exhibiting a long life has been developed. If the foregoing technique for detecting the density is employed, the developer remains adjacent to the detecting surface of the toner-density sensor or toner adheres to the detecting surface of the transmission after the apparatus has been used for a long time. Thus, an error occurs in detecting the density of toner. As a result, the density cannot stably be detected for a long time.